Fluid Flow - Latest Discussions - COMSOL Forums

Fixed Temperature Boundary Condition

Tue, 22 May 2012 13:31:03 +0000

Hello, I'm new to COMSOL and am trying to run a non-isothermal flow problem where I have basically a rectangular solid object immersed in a volume of air flowing across the object. I placed a fixed temperature boundary condition on one of the sides of the rectangle, a temperature and velocity boundary condition on the inlet and a pressure boundary condition on the outlet. What I'm trying to accomplish is to determine the heat loss from the object to the air (without radiation for the time being). After I run the scenario, I plot the temperature and to my surprise the temperature on the wall that I placed the fixed boundary condition on is somehow lower than the fixed value I applied. What have I done wrong, or is there something else I need to do to the model?

Thanks

Cotinuous Release in Particle Tracing Module

Tue, 22 May 2012 13:27:54 +0000

I would like to continuously release particles for a set amount of time using the new particle tracing module. Under hte Inlet settings there only seems to be the availability of step-wise release times. Is there any way to implement continuous release in this module? Or should I so a quasi-continuous release, ie: setting the interval time to a very small value?

Fluid Shear Stress in Thin Film Fluid Module

Thu, 17 May 2012 13:49:04 +0000

Hello,

I'm currently working with the journal bearing model (Comsol 4.2) from the model library and I'm trying to calculate the shear stress within the fluid film with is an essential for the torque calculation. Unfortunatly the shear stress is not one of the output values of this module.

I tried the surface integration and using the simple formula:
tau = eta*du/dh
with dh = tffs.dh
and eta = constant value

I used du = d(tffs.uy,y)+d(tffs.ux,x)

Unfortunately the results don't seem logical. I believe my expression for du or my usage of the integration (under results, derived values) is wrong.

Please help!

Thanks,
Toan

finding a specified flow rate in a '3D cut plane'

Tue, 15 May 2012 20:08:55 +0000

Hello,

I'm trying to do some postprocessing in a 3D fluid channel flow over a fractured hole.
The channel is filled with water and is laminar.
Here's what I'm trying to do:

1. cut the surface at an arbitrary channel location (i used the 'cut plane')
2. within that surface, I would like to start from one edge and integrate the velocity profile in a prescribed area.
eg) if the surface slice is a 2mmx2mm rectangle, i want to make another rectangle that is 1mmx1mm within that original 'cut plane' and measure the surface integrated velocity value (flow rate).

I was able to cut the surface where I wanted it, but I cannot find a way to resolve what I said in 2.

Any ideas?

Thanks!

TJ

Moving Interior Wall in cdf

Tue, 15 May 2012 14:25:58 +0000

Hi,

I am studying pipe flow, currently in 2D.

I have placed a rigid interior wall, making a certain angle with the direction of flow, inside the pipe. Ideally, I would like it to rotate 'freely' around its first edge (see picture, the red point would be the axis). As of now, I am only using the Laminar Flow module.

By freely, I mean subjected to the flow. How can I achieve this?

1) the freely rotating axis
2) make the mesh follow the interior wall

I am reading this http://www.comsol.com/support/knowledgebase/970/ and looking for examples.

Thank you

Simple convection model not converging

Tue, 15 May 2012 13:12:31 +0000

Hello all,

I have a simple beam model in which the beam is made out of silicon and it is sorrounded by air. When I apply a voltage to the beam, it creates temperature gradient along the beam due to the resistivity of the beam being dependent on the temperature at which it is at. To incorporate the convection due to the air, I went ahead and used two physics to mimic this phenomenon: electric currents (to compute the resistance of the beam and the power) and non-isothermal flow (to compute the temperature, including the effect of convection). The model itself is a one quarter model, allowing for symmetry, which I took care of with the simple boundary condition of electric and thermal insulation. The reason behind coupling these is so that I can see the effect of convection, given the fact that the heat transfer coefficient of air will change due to temperature. When I run my model with time dependent I get the following error:

"Failed to find consistent initial values.
Last time step is not converged."

When I run it with stationary I get the following error:

"Failed to find a solution.
Maximum number of segregated iterations reached.
Returned solution is not converged."

Any help is greatly appreciated.

Thanks.

Please give me some hints for a high Mach number turbulent modeling

Mon, 14 May 2012 16:59:28 +0000

Hello advanced users,

I am trying to model a fluid flow in a nozzle.
As there were too many error with the initial conditions, (e.g. Attempt to evaluate real square root of negative number...)
I referred some initial values from a tutorial 'Sajben diffuser'.
After input the values, that error was solved, but still there is a problem with

''Undefined value found.
Detail: Undefined value found in the equation residual vector.
There are 57 degrees of freedom giving NaN/Inf in the vector for the variable mod1.nutilde''

When I checked the log file, a variable 'mod1.nutilde' seems to be diverging, and it makes the error.
How can I fix this error?
Thanks a lot in advance.

Regards
MJ

Turbulence in pipe flow

Fri, 11 May 2012 11:43:24 +0000

Hi,

I am studying the Navier Stokes equation in the context of pipe flow. As of now, I am using Water, modeling it as incompressible, with outlet at 0 Pa and a parabolic inlet profile. (currently at 2D, but moving to 3D as things advance).

I am currently using Fluid Flow>Single-Phase Flow>Laminar Flow, as my inlet is a stable velocity profile and the turbulence, if any, will arise due to obstacles inside the pipe.

I have been playing with it, doing the Von Karman vortexes simulation (http://www.comsol.com/showroom/gallery/361/), looking into a moving mesh (http://www.comsol.com/showroom/gallery/361/), as my object might be able to move subjected to the flow.

My question is: how far can I push the Laminar Flow module? I am looking for turbulence, so I don't think I should be using a Turbulence specific model (it forces turbulence, and requires me to input parameters specific for turbulence, that clearly doesn't exist at the start). On the other hand, I don't know if the Laminar Flow module will be able to propely simulate turbulence.

What should I be using then?

Thank you for input.

Thermal Dispersion in a Poreus Medium Module Problem

Thu, 10 May 2012 11:33:25 +0000

Hi,

I am working on simulation an Aquifer Thermal Energy Storage (ATES) [1] system. In this I am trying to use thermal dispersion, which is part of the heat transfer module. However I cant get it to work. I have tried the manual and the internet, but I cant figure out what the problem is and let alone solve it. Maybe one of you guys can see what the problem is, and how I would be able to solve it?

I've included the error log and the model, which is a 4.0a model.

Greetings Nick

[1] http://www.iftech.co.uk/index.php?option=com_content&task=view&id=21&Itemid=33

Attempt to evaluate negative power of zero.
- Function: ^
Failed to evaluate temporary symbolic derivative variable.
- Variable: mod1.ht2.k_dispxx@VDN$mod1.py
- Defined as: (((1/max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps))*((mod1.ht2.rho*mod1.ht2.Cp)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2))))+((((((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))^-0.5)*0.5)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2)))*(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2)))>eps))*((-((mod1.ht2.rho*mod1.ht2.Cp)*(((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))))/(max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps)^2))))
Failed to evaluate temporary symbolic derivative variable.
- Variable: mod1.ht2.k_eqxx@VDN$mod1.py
- Defined as: ((((1/max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps))*((mod1.ht2.rho*mod1.ht2.Cp)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2))))+((((((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))^-0.5)*0.5)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2)))*(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2)))>eps))*((-((mod1.ht2.rho*mod1.ht2.Cp)*(((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))))/(max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps)^2)))))
Failed to evaluate temporary symbolic derivative variable.
- Variable: mod1.ht2.k_effxx@VDN$mod1.py
- Defined as: (((((1/max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps))*((mod1.ht2.rho*mod1.ht2.Cp)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2))))+((((((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))^-0.5)*0.5)*(((((1/(mod1.dl.g*mod1.dl.rho))*(-mod1.dl.Kyy))))*(mod1.ht2.uy*2)))*(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2)))>eps))*((-((mod1.ht2.rho*mod1.ht2.Cp)*(((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))))/(max(sqrt((((mod1.ht2.ux^2)+(mod1.ht2.uy^2))+(mod1.ht2.uz^2))),eps)^2))))))
Failed to evaluate expression.
- Expression: d(d((-mod1.ht2.k_effxx*mod1.T2x*test(mod1.T2x)-mod1.ht2.k_effyy*mod1.T2y*test(mod1.T2y))*(dvol),{test@3}),mod1.py)

Boundaries that change their character in time

Thu, 10 May 2012 09:25:07 +0000

Hi all
Can I change the property of a boundary between an inlet to become a wall (no slip) depending on time?

I'm trying to model a contracting heart. I managed to move the boundaries and to make the flow move with them as well. The problem is with the inlet and outlet. The thing is that they are both valves, which means that while the heart contracts the inlet is a closed wall (no slip boundary) and the outlet is a (let's say) 0 pressure outlet. When the heart expands the outlet becomes a closed wall (no slip boundary) and the inlet is now an inlet with (let's say) 100 Pa pressure.

Any way around this?

Thanks a lot
Hemi

simulate two air bubbles intergrating in the water

Wed, 09 May 2012 13:55:09 +0000

what are the steps should I do to simulate two air bubbles intergrating in the water using comsol 4.2 ?
I am firstly contacting with comsol 4.2. Thank you.
Should I use Two phase-flow ?

Air flow through elbow channel

Wed, 09 May 2012 08:38:40 +0000

Hi all!

I have problem with simulation of flow of air through the elbow channel. I have the box 500x50x80mm with the fi 50mm inlet from 50x80mm side and whole base (500x50) is an outlet with grill which should ensure flow only in normal direction (see attached model).

The task I am facing is to obtain the pressure distribution at the outlet.

I think my problem is connected with proper meshing of the problem and setting the boundary conditions, but still I cannot obtain any solution.

The usual errors I get are that the limit of Newtonian iterations have been reached.

Could you help me with making this model to run?

Best regards,
Michał Radziwon

turbulent flow through elbow channel

Tue, 08 May 2012 12:55:36 +0000

Hi all!

I have been challenged with optimisation task which deals with turbulent flow of air/gas mixture through an elbow.

There is a box 500x50x80mm with grill at the bottom, which ensures only normal outflow of the fluid. The inlet is through fi 50mm tube from the side - with average velocity of 5.9m/s (check attached geometry in the file).

The task is to find the placement of some kind of orifice to equalize the outlet velocities through the outlet.

The problem is that I am unable even to obtain a solution with no orifice at all. The common errors I get are - out of Newtonian iterations, or problems with meshing.

I would be grateful if somebody could help me to obtain a working simulation of this, as it seems, rather easy problem.

Best regards,

Mike

shear stress in microchannel

Tue, 08 May 2012 02:50:24 +0000

Hello,

I am clueless as to how to start the post-process data analysis to get the spatial distribution of shear stress of a micro channel. I have read the few discussion threads but it involves exporting the slice 3D plot of the velocity and then do the calculation of shear stress in Matlab. I would appreciated it if someone can either tell me (a) what Matlab code or equation you used to solve for shear stress from the velocity profile slice or (b) a way to get Comsol to compute shear stress for me and graph it (a much easier way for a newbie like me).

Thank you so much!

non isothermal heat transfer in room

Mon, 07 May 2012 14:42:34 +0000

Hello,

I am trying to model a simple room 4 m x 3 m, 2.90 m high with:
- open door (to large volume of air) - outdoor = open boundary, 20°C
- open window (to large volume of air) - indoor = open boundary, 20°C
- a heat source in the room = gas furnace, 100 m3/hr of fluid, temp 1000°C = inlet, laminar inflow 100 m^3/hr, temperature 1000°C
- gravity (-g_const*nitf.rho), defined as volume force

Physics is nitf, non isothermal transfer in fluid.

My problem is that it never converges although I reduced and improved a lot mesh size.
If I change the "open boundary" condition with "outlet", it can converge but setting a temperature at the outlet is not realist.

Does air density variation with temperature is automatic in 4.2 o do I have to introduce a formula to take this in account?

Thanks for any help you can provide,

Pedro

weak constraint or pointwise constraint

Mon, 07 May 2012 01:21:31 +0000

Hello,

I would like to have ur opinion for this situation:

I wish to constraint the (fluctuating) pressure to an average value of 20 [constraint expression: aveop1(p)-20] at the inlet boundary while pulsatile flow velocity is imposed at the inlet too.

I am not sure if weak constraint or pointwise constraint option is more suitable for this condition?
I read that weak constraint and pointwise constraint are similar but there is no Lagrange multipliers created in pointwise constraint. Anyone can please tell me what the importance of Lagrange multipliers in this case?

Thank you for your kindness.

Mass balance problem when using user defined density.

Sun, 06 May 2012 22:12:06 +0000

I used user defined density in a Darcy's Law model because the temperature is very low (35 K for hydrogen) but there is mass balance problem (more than 10%) while when I used ideal gas as the mass balance is ok.
I have checked the density I defined there is no problem for this part actually the density is obtained from NIST/REFPROP through MATLAB.

Conjugate, non-isothermal u-pipe, time-dependent study

Tue, 01 May 2012 19:04:23 +0000

I wanted to simulate the heat transfer of fluid in a u-pipe. However, I'm really confused about which module and model should I use?

Initially what I wanted to do is to choose the heat transfer in porous media modulue, however, I found difficulties of solving the problem of directing the fluid from vertical pipe into u-shaped corner by defining a velocity. Based on my shallow experience with Comsol so far, it seems that this modulus only allows the fixed velocity vector to be defined at the input stage, and I found it difficult to resolve for the model.

I have also tried to use the conjugate heat transfer as well as non-isothermal flow... then I started to get more confused because my analysis has to be carried in time-depenent study. Would anyone give me some advices?

Thanks for your advice in advance!

Need some help in meshing, inverted mesh elements warnings

Tue, 27 Mar 2012 07:23:37 +0000

I have an FSI problem, by moving a membrane I would like to see the fluid move.

When I setup the simulation, I've played around with the settings, looked at how other models were setup, but I can't get rid of the 'inverted mesh elements' warnings.

Does anyone have any advice on this model?

looking for help on modeling an welding arc

Sun, 18 Mar 2012 13:10:08 +0000

Hello!

My name is Rainer, im new to the world of comsol. I did FEM and CFD on Siemens NX. I could use some help on modeling an welding arc (temperature field, magnetic field and flow in the melted steel).

Me and my team is building an welding tractor or robot or whatever you want to call it for my final exams. it will follow undefined contures by magnetic forces from permanent magnets.

Now i need to prove that the permanent magnets dont affect the welding arc. (magnetic blow...)

thanks!

take care,

Rainer